Gas turbine engines are widely used to generate power for numerous applications. A conventional gas turbine engine includes a compressor, a combustor, and a turbine. In a typical gas turbine engine, the compressor provides compressed air to the combustor. The air entering the combustor is mixed with fuel and combusted. Hot gases of combustion are exhausted from the combustor and flow into the blades of the turbine so as to rotate the shaft of the turbine connected to the blades. Some of that mechanical energy of the rotating shaft drives the compressor and/or other mechanical systems.
As government regulations disfavor the release of nitrogen oxides into the atmosphere, their production as byproducts of the operation of gas turbine engines is sought to be maintained below permissible levels. Fuel-air mixing affects both the levels of nitrogen oxides generated in the hot gases of combustion of a gas turbine engine and the engine's performance. A gas turbine engine may employ one or more fuel nozzles to intake air and fuel to facilitate fuel-air mixing in the engine's combustor. The fuel nozzles may be located in a head end portion of the gas turbine engine, and may be configured to intake an air flow to be mixed with a fuel input. Typically, each fuel nozzle may be internally supported by a center body located inside of the fuel nozzle.
Various parameters describing the combustion process in the gas turbine engine correlate with the generation of nitrogen oxides (NOx). For example, higher gas temperatures in the combustion reaction zone are responsible for generating higher amounts of nitrogen oxides. One way of lowering these temperatures is by premixing the fuel air mixture and reducing the ratio of fuel to air that is combusted. As the ratio of fuel to air that is combusted is lowered, so too the amount of nitrogen oxides is lowered. However, there is a trade-off in performance of the gas turbine engine. For as the ratio of fuel to air that is combusted is lowered, there is an increased tendency of the pilot flame of the injector to burn out and thus render unstable the operation of the gas turbine engine. So-called Lean Blow Out (LBO) events, which are characterized by extinguished flames due to an air/fuel mixture that is too lean (insufficient fuel), increase emissions and reduce combustor efficiency.
U.S. Pat. No. 6,446,439, which is incorporated in its entirety herein by this reference for all purposes, injects fuel into an annular passage within the center body where mixing with air occurs, and the premixed mixture of air and fuel is then swirled and injected as a swirling pilot. However, combustion stability at very low levels of NOx emissions, i.e., below 3 parts per million (ppm), cannot be achieved in this manner.
Thus, a need exists for combustion stability at very low levels of NOx emissions, i.e., below 3 parts per million (ppm). In order to achieve very low levels of NOx emissions with some margin of error and non-uniformity around the turbine, stable operation (i.e., greatly improved avoidance of LBO) of the fuel injector is required.